1. Field of the Invention
The present invention relates to methods and apparatus for cutting semiconductor wafers and wafer scale assemblies for singulation into dice or groups of die locations. More particularly, but not limited thereto, the present invention relates to the singulating of semiconductor wafers and wafer scale assemblies using laser cutting, alone or in combination with other techniques, to enable a reduction in the required width of streets between semiconductor die locations.
2. State of the Art
Singulating semiconductor wafers, also known as dicing or die separation, is the process of cutting a semiconductor substrate having integrated circuits formed thereon into individual semiconductor dice. Currently, although a number of methods for singulating semiconductor wafers are known, the most commonly used methods involve cutting the wafer along scribe or severance lines (commonly termed “streets”) with a rotating circular abrasive saw blade.
Dicing with a rotating blade is a mechanical process of machining with abrasive particles to remove material from selected areas of the wafer. Dicing saw blades are typically made in the form of an annular disc that is either clamped between the flanges of a hub or built on a hub that accurately positions the thin flexible saw blade, which carries diamond particles as the abrasive material. These saw blades are extremely thin (typically from 0.6 to 50 mils in thickness), flexible, rotate at high speed and are, therefore, susceptible to damage from binding due to deviations in the cutting path during singulation, leading to complete blade failure and usually substantial damage to the wafer being singulated.
Semiconductor wafers are coated with various layers in the course of fabricating integrated circuitry thereon at die locations, such coatings including passivating oxides or nitrides, polymer coatings, aluminum and a wide variety of other metals, in recent years including copper. Wafer streets may include at least some of these coatings as alignment markers. Further, test elements for use in probe testing of wafers may be placed in the areas within the streets to conserve wafer space (or “real estate”) and enable more semiconductor dice to be formed on each semiconductor wafer for a given wafer size. Wafer streets are usually fully or partially coated with different materials and are largely nonhomogenous. The combination of materials in the streets, combined with the hardness of the underlying semiconductor material of the wafer substrate (typically crystalline silicon, although other materials such as germanium, gallium arsenide and indium phosphide are also employed), has a significant effect on the wafer singulation process and affects resulting die edge quality. In conventional dicing using a wafer saw, the die surfaces at the bottom or “back” side of the wafer can suffer severe bottom-side chipping. On the top or “active” surfaces of the dice on which the integrated circuitry is fabricated, conventional dicing can lead to cracking of the material layers, smearing and tearing of metallic pads, formation of polymer slivers, and shredding and tearing of the semiconductor material of the wafer substrate. For this reason the streets must be of sufficient width to allow any such anticipated damage occurring during dicing to remain clear of the functional integrated circuit elements of the dice, consequently reducing the real estate available for forming semiconductor dice on the wafer.
Attempts have been made to minimize the damage occurring during dicing. For example, lasers have been used to singulate wafers, although the heat generated by a laser cutting through a thick wafer can thermally stress and damage both the semiconductor material of the resulting semiconductor dice and integrated circuit components thereof. Lasers may also produce slag from melted material lying along cut edges that may contaminate the semiconductor dice. Conventional laser cutting, like conventional wafer sawing, thus similarly requires streets to be of sufficient width to allow any damage occurring during dicing to be confined to areas of the semiconductor dice clear of the integrated circuit elements thereof, again reducing the real estate available for forming dice on the wafer.
Another attempt to minimize dicing damage is to make a partial cut through the wafer with a beveled saw blade, followed by a second cut passing through the wafer within the partial cut with a narrower saw blade. While such a technique reduces the incidence of bottom or back side chipping by reducing the width of material made by the final cut, it fails to remedy the other problems associated with abrasive cutting and may even require wider streets for the beveled blade. U.S. Pat. No. 6,420,245 to Manor, the entire disclosure of which is incorporated herein by reference, discloses an attempt to combine this technique with laser cutting by making a shallow cut of the top surface with a laser to form scribe lines prior to cutting with an abrasive rotating blade. This technique, however, requires the use of both a laser and a mechanical saw, potentially exposing the singulated semiconductor dice to damage associated with both of these techniques and requiring streets wide enough to absorb such damage.
An alternative method of dicing is disclosed in U.S. Pat. No. 6,136,668 to Tamaki et al. Using this technique, a metal layer, in the form of a grid extending over the streets, is formed over an active surface of a silicon wafer. The silicon wafer body is singulated, but for the presence of the metal layer, by etching therethrough (after backgrinding to reduce the wafer thickness) from the back side to the metal layer. The dice are then separated by melting and fusing the metal layer along the streets using a laser. Since this method involves etching through the entire thickness of the wafer and requires the presence of the metal layer as well as enough separation from the integrated circuitry of each semiconductor die to avoid damage from the laser-fused metal, the required width of the streets remains significant. This method also requires the extra steps to form the metal layer.
Accordingly, an approach to singulation of a semiconductor wafer which enables the use of smaller street widths would be an improvement in the art. Similarly, such an approach additionally offering the capability of producing semiconductor dice with protected edges at the perimeter of the active surface would also be an improvement in the art.